This invention relates to soft hydrogel contact lenses, made of polyoxirane crosslinked polyvinyl alcohol, which are swollen but do not dissolve in water.
In general, most existing hydrogel soft contact lens materials are based on HEMA, also known as hydroxyethylmethacrylate or as ethyleneglycolmonomethacrylate, with one or more optional comonomers as described in U.S. Pat. Nos. 2,976,576, 3,841,985 and 3,985,697. Other hydrogels based on N-vinylpyrrolidone copolymers and acrylamide copolymers are disclosed in U.S. Pat. Nos. 3,639,524 and 3,929,741. These prior art hydrogel polymers suffer from several inherent problems: (a) all contain hydrolyzable ester or amide linkages, (b) all may contain toxic residual monomers or oligomers that can be released into the eye, (c) most lenses are made by either of two procedures, one requiring costly lathing and polishing steps and the other using delicate spin casting techniques where polymerization, crosslinking, and shaping are done simultaneously, (d) all offer, due to the presence of ester and especially amide linkages, sites for enzymatic attack initiating a hydrolytic process or antigen-antibody reactions (i.e. deposition), and (e) most do not have sufficient dissolved oxygen permeability to prevent corneal edema with the consequent risk of permanent eye damage.
It is an object of the present invention to provide soft contact lenses obviating or substantially reducing the aforementioned drawbacks of the prior art.
Polyvinyl alcohol, also known as PVA, films and gels have been reported as opthalmic inserts in the lower conjunctival sac when imbibed with antibiotics such as tetracycline, pilocarpine, atropine and the like. Such materials are either in the form of a crosslinked film or as a water soluble viscous solution or gel. See, for example, Y. F. Maichuk, Ophthalmic Drug Inserts, Invest. Ophthalmol., Vol. 14, pages 87-90 (1975); D. W. Lamberts, Solid Delivery Devices, Int. Ophthalmol. Clinic, Vol. 20, No. 3, pages 68-69 (1980) and Y. F. Maichuk, Antibiotik, Vol. 12, No. 4, pages 432-5 (1967).
Many reagents have been suggested for crosslinking PVA--these include organic and inorganic reagents as well as radiation (.gamma. or X-rays).
For example, Tanaka, Japanese Patent 72/06910, describes a method of making a thick block of crosslinked PVA by heating together PVA and formal in using acid catalysis. The key step in this procedure is to simultaneously dry and heat the mixture using a water vapor-porous paper mold until a crosslinked hard block of PVA results which, for example, can be molded and then swollen in water to form a contact lens. There are several problems with this procedure and the resultant material:
1. The drying process induces crystalline regions into the PVA matrix in an uncontrolled fashion. Once swollen or boiled in water, the material experiences non-reproducible dimensional increases.
2. This crosslinking reaction requires fairly high temperature (.about.130.degree. C.) sufficient to initiate an acid catalyzed dehydration reaction along the PVA backbone. The resulting conjugated double bonds gives rise to a yellow color in the PVA matrix.
3. The crosslinking of PVA with formaldehyde results in an acetal linkage which is readily reversible under aqueous pH conditions, especially under the temperature conditions used for heat sterilization.
4. The processing, i.e. machining and polishing, of the PVA block as with other xerogels is expensive and time consuming.
The Japanese patent application 50/115258 discloses a process and material that may have contact lens use. A dialdehyde of PVA is mixed with PVA and an acid catalyst with the solution then coated on a glass plate, air dried and then heat treated at .about.80.degree. C. This process also requires a drying step and high temperature to force the crosslinking reaction. As with the Japanese patent 7206910 one has problems with induced crystallinity and also the reversible acetal crosslinkage.
In U.S. Pat. No. 3,232,916, crosslinked PVA for battery separators was made by dissolving PVA and a polyoxirane in water with an acid catalyst spreading the mixture upon a smooth surface, evaporating off the water at room temperature and then crosslinking at high temperature. Although this material may be useful for ion permeable battery separators it would be undesirable in contact lens use, as the water evaporation step induces crystallinity and the subsequent crosslinking leads to irreproducible dimensional changes upon heat sterilization. Also, the high temperature employed to effect crosslinking colors the material by a dehydration mechanism. Thus, the patent's example 2, using an amine catalyst, results in a golden colored film.
The Japanese patent 49/35466 eliminated the difficulties arising from a drying step by placing the aqueous PVA solution into a sealed mold. However, the actual crosslinking agent used presents various problems. For example, by their using 0.4% PVA in water and using .gamma. or X-rays to crosslink the PVA necessitates long irradiation times. The radicals that are produced not only crosslink PVA but also cause its degradation. The resultant material tends to be weak and brittle, not only due to the short length between crosslinked portions (.about.1.5 .ANG.), but also due to the fragmented nature of the crosslinked chains. In this patent's other crosslinking procedure, PVA was mixed with polyacrylic acid under acid catalysts and heated for 2 days. The resultant crosslinkage produced is a hydrolyzable ester. In addition, the unreacted carboxylic acid groups present render the dimensional changes of the final hydrogel sensitive to pH fluctuations in the tear fluid. Thus, not only is the final material not stable to the pH environment of tear fluids but undesirably long reaction times are necessary to form the gel.
Polyvinyl alcohol crosslinked with glyoxal has been proposed as a contact lens material, e.g. in U.S. Pat. No. 3,408,429. Unfortunately, the acetal and hemiacetal group formation which results from the crosslinking reaction is reversible under mildly acidic conditions, resulting in the potential release of glyoxal from the crosslinked material. The reversibility increases greatly under temperature conditions used for heat sterilization. Glyoxal is known to be an irritant to skin and mucosa. Also, no disclosure of PVA molecular weight and no specifics concerning the amount of water present in the crosslinking step are disclosed.
It is an object of the present invention to provide soft contact lenses obviating or substantially reducing the aformentioned drawbacks of the prior art.
It is a further object of the present invention to provide contact lenses comprising polyvinyl alcohol which have been crosslinked with a multifunctional oxirane compound such that the crosslinked lenses are insoluble in the ocular tear environment, can be boiling water sterilized, possess a high water content, high dissolved oxygen permeability, low protein absorption and good mechanical strength. Lenses of this type also are of value as an "eye bandage" for non-severe corneal damage.
It is a further object of the invention to provide rapid, simple low cost molding processes for the preparation of such lenses.
Many available soft contact lenses are easily damaged being fragile, easily scratched and torn. There is a constant risk of tearing the material with a finger nail when inserting or removing from the eye. They also must be frequently sterilized to prevent bacterial growth in the hydrogel network and constantly cleaned to remove all types of deposits e.g. protein, lipids, and calcium salts. These deposits cloud the lens and become a serious source of eye irritation e.g. Giant Papillary Conjunctivitus.
The instant inventive contact lenses have the additional advantage in that they are economically prepared and thus can be discarded at regular intervals, i.e. disposable.